1. Field of the Invention
The present invention relates to a process for cleaning a flue gas containing SO.sub.x, NO.sub.x and O.sub.2 by washing it with an alkaline aqueous solution containing a NO.sub.x removal catalyst.
2. Description of the Prior Art
When the flue gas containing SO.sub.x, NO.sub.x and O.sub.2 is washed with an alkaline aqueous solution containing a NO.sub.x removal catalyst, an oxidation is caused as well as the SO.sub.x removal and the NO.sub.x removal to obtain a byproduct containing a dithionate, a sulfate and an imidobissulfate as main components.
The main reactions resulting in the simultaneous SO.sub.x removal and NO.sub.x removal by using a base are as follows: The base include hydroxides or carbonates of potassium, sodium, magnesium, calcium or ammonium.
SO.sub.x removal EQU H.sub.2 O+SO.sub.2 .fwdarw.SO.sub.3.sup.2- +2H.sup.+
NO.sub.x removal EQU 5SO.sub.3.sup.2- +2NO+3H.sub.2 O.fwdarw.SO.sub.4.sup.2- +2NH(SO.sub.3).sub.2.sup.2- +4OH.sup.-.
In said reactions, 2.5 mol. of sulfite (SO.sub.3.sup.2-) is required to absorb 1 mol. of NO gas.
On the other hand, most of the sulfite produced by absorbing SO.sub.2 from the flue gas is converted into polythionates such as S.sub.2 O.sub.6.sup.2- and S.sub.n O.sub.6.sup.2- by an oxidation with O.sub.2 gas in the flue gas or a side reaction in the washing solution.
A large amount of dithionates is produced as the main oxidized products by the following reaction in the presence of a Fe catalyst. EQU Chelate Fe(II)+1/4O.sub.2 +1/2H.sub.2 O.fwdarw.Chelate Fe(III)+OH.sup.- EQU Chelate Fe(III)+SO.sub.3.sup.2- .fwdarw.Chelate Fe(II)+1/2S.sub.2 O.sub.6.sup.2-.
Two mol. of a dithionate is produced by the reaction of 1 mol. of O.sub.2 with consumption of 4 mol. of sulfite (SO.sub.3.sup.2-).
(All of O.sub.2 is not always reacted, but only some of O.sub.2 is reacted.)
When the flue gas containing 3 to 3.5 vol.% of O.sub.2, 1600 ppm of SO.sub.2 and 280 to 300 ppm of NO was washed with an alkaline aqueous solution as the absorbent to absorb 80 to 90% of NO, a consumption of the sulfite for producing the dithionate is ranging from 4.5 to 6.5 mol. during absorbing 1 mol. of NO. This is only one example. Such large consumption of SO.sub.3.sup.2- is found. The consumption of the sulfite caused by the production of the dithionate is depending upon a concentration of O.sub.2 in the flue gas, a NO.sub.x removal efficiency and catalytic characteristics in a NO.sub.x removal absorbent solution.
Even though a molar ratio of SO.sub.2 /NO is 5.5, the shortage of the sulfite for the NO.sub.x removal is caused, that is, the amount of SO.sub.2 to the amount of NO.sub.x is not enough whereby a desired NO.sub.x removal rate has not been attained. In order to overcome the shortage of SO.sub.2 gas, it has been proposed to add a strong acid to an aqueous solution containing the dithionate and the polythionate and then to heat it at high temperature so as to obtain SO.sub.2 gas as the decomposed product.
In this process, a large amount of a strong acid should be used at high temperature under high pressure for a long time to cause serious problem for selection of material for the apparatus. Moreover, in order to treat excess of the acid, a complicated and disadvantageous operation such as excess acid neutralization with a base has been required.
As a result, the consumptions of the acid and the base are obliged to increase and the formation of a large amount of valueless products such as gypsum, ammonium sulfate and sodium sulfate are disadvantageously increased.